1. Field of the Invention
This invention relates to a storage plate for signal storage tube for high speed writing, for use in the construction of for example, a vidicon tube having a beam generating system for producing an electron beam for writing, reading and erasing and a central deflecting device, in which tube there is generally provided on a conducting signal plate thin insulating spots (thin layers) of high secondary emission and/or low capacitance uniformly distributed in the form of a raster. This invention also is particularly concerned with the process for producing the signal plate.
2. Description of the Prior Art
A storage tube already known in the prior art and generally called a "lithocon" tube utilizes a silicon plate having a uniformly distributed raster of approximately 2000 .times. 2000 silicon dioxide spots arranged thereon as a storage medium. The resulting capacity of this tube permits aimed writing, reading and erasing with one and the same electron beam. The formation is then read without interference, the reading time with continuous reading amounting to approximately one hour. The recharging time for an image point is approximately 30 n sec. so that relatively high writing speeds are attainable.
The foregoing type of tube has special storage possibilities for gray values. The operating voltage is, in comparison to other known corresponding storage tubes, relatively low, having a maximum operating voltage of about 1000 volts. In this type of storage tube, the insulating spots are of SiO.sub.2 arranged on a silicon signal plate having thickness up to 1 .mu.m, because the oxidation process to provide greater thicknesses would require intolerably long times. In the scope of the functioning mechanism, the storage plate behaves as the determinative part of a triode, in which the surface potential of the storing insulating layer serves in a certain sense as the grid and the substrate which has good conductivity takes over the function of the anode. With such an arrangement it is possible to reproduce halftone images as long as only the potential of the storage islands (spots) is constantly more negative than the place of origin of the scanning electrons, that is, the cathode of the beam generating system.
The foregoing type of tube has, however, a serious drawback with respect to the magnitude to the writing speed to be achieved. This writing speed is defined by the expression ##EQU1## in which J.sub.p signifies the beam current, .delta. the secondary emission factor of the storing surface, C the capacitance of a storage element and .DELTA. U the potential change impressed by the writing on the storage element.